Heretofore, inorganic glasses having a superior heat resistance and optical properties have been used for optical plates and lenses, but such inorganic glasses are disadvantageous in that they are easily broken because they have a poor shock resistance and are heavy due to a large specific gravity thereof. Efforts have been made to overcome the drawbacks of inorganic glasses by substituting resins for the inorganic glasses, and as such thermoplastic resins, polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), polycarbonate (PC), and polyether sulfones (PES) may be cited for example. These resins, however, invariably have a critical problem which hinders the use thereof as sheetlike articles for optical applications. The PMMA, for example, has a disadvantage in that it has too low a heat resistance to withstand the conditions involved in the imparting of an electroconductive transparent film thereto, and after the imparting of the electroconductive film thereto, exhibits a low heat resistance such that the transparent electroconductive film is easily cracked during the fabrication process. The PET, PC, and PES, though having a relatively good heat resistance, have too high a double refraction index to be used advantageously, particularly for sheetlike articles in optical applications such as, for example, substrates for liquid crystal display devices and substrates for optical recording discs.
Thermoplastic resins such as diethylene glycol bisallyl carbonate polymer (CR-39) have been used for the same purpose, and not unexpectedly, they have a poor heat resistance, and particularly, have low glass transition points, and therefore, cannot provide sheetlike articles giving fully satisfactory performances for optical applications.
Japanese Unexamined Patent Publications No. 131,654/1981 and No. 126,119/1983 disclose substrates made of polymers using maleimide compounds and intended for use in optical discs, but these techniques are disadvantageous in that the substrates produced thereby have a poor resistance to solvents and chemicals.